Development of efficient electric motor drives for vehicles, with improved electronic control systems and portable power sources, has drawn increasing attention as a viable alternative or adjunct to combustion engine drives. For example, U.S. Pat. No. 6,492,756 to Maslov et al. and U.S. Pat. No. 6,617,746 to Maslov et al. describe motor structures that provide high torque output capability with minimum power consumption that are especially suitable to electric vehicle propulsion. Electromagnets are used as isolated magnetically permeable structures configured in a stator annular ring. Isolation of the electromagnet groups permits individual concentration of flux in the magnetic cores of the groups, with virtually no flux loss or deleterious transformer interference effects with other electromagnet members. Operational advantages are gained from this segmented electromagnetic architecture. Magnetic path isolation of the individual pole pair from other pole groups eliminates a flux transformer effect on an adjacent group when the energization of the pole pair windings is switched.
Electric traction systems demand high torque from low voltage propulsion units. The low voltage restriction satisfies a need to conserve space by minimizing the number of battery cells and eliminating extra insulation that otherwise would be required for high voltage protection. In order to deliver high torque from a low voltage source, it is necessary to draw high current through the motor windings. The windings must be thick to provide the high current capability. The use of a thick single wire to form a stator winding typically introduces unacceptable high frequency skin effect losses. To reduce such losses it has been customary to use multiple strands of thin wires in a bundle to provide the required coil thickness. The individual strands can share the current load while reducing skin effect losses. The bundling of such wires is commonly known as “wires in hand.”
A conventional automated technique for bundled multiple stranded stator winding formation is illustrated in the schematic diagram of FIG. 1. A four wires in hand process is exemplified. Each wire strand 10 is supplied by an individual wire spool 12. All strands are fed simultaneously to wire bundler 14, which combines the strands into a single bundle 16 of four wires in hand. The bundle is then wound on each stator pole 18 of stator assembly 20. A human operator guides the thick wire bundle around the slowly rotating stator assembly. Several drawbacks can be attributed to this technique. As one spool of wire is required for each strand, a winding system that must accommodate an increased number of strands becomes complex. Bending a thick bundle of wire on a stator is difficult while maintaining the high tension required from the winding machine. The “slot fill factor,” which is a measure of the amount of the slot volume occupied by the winding, is reduced as the thickness is increased. These factors seriously limit the number of strands that can be bundled in the conventional winding construction.
The need thus exists for a thick stator winding that can provide maximum slot fill factor with uniform inductance and resistance characteristics. A winding technique is needed that would provide such a winding that has no inherent limitation on the winding thickness and can provide appropriate bending with a minimum of complexity.